In recent years, the need for additional oil, gas and other mineral resources has brought about increased activity in the exploration for and recovery of such resources from offshore locations. In order to perform the necessary exploration drilling, production drilling, and in some instances petrochemical-processing, it is necessary to provide a stable platform structure from which such activities can be conducted. At locations having small or marginal offshore reservoirs, the approach taken in recovering the minerals has been to erect mobile offshore jack-up platforms that permit a single platform structure to be utilized at several successive reservoirs.
A typical jack-up unit has a buoyant hull and has separate support legs that project upwardly from the hull during transport. Once the unit has reached the desired location, the support legs are lowered into contact with the ocean floor and the unit is raised to a level above the surface of the water. When operations are finished at a particular location, the structure can be moved to another site.
The increased activity in the exploration for and recovery of oil and gas from offshore locations and the mobility of jack-up rigs, has led to an increase in offshore drilling sites. This increase in offshore drilling sites increases the potential for offshore drilling equipment to be damaged due to environmental factors, especially in areas prone to revolving tropical storms such as the Gulf of Mexico and southeast Asia.
A jack-up unit may be lost or damaged by three main types of potential accidents, which include: (1) overturning when the weight of the unit cannot counterbalance the overturning moment due to environmental loads; (2) soil failure under a leg resulting either from sudden penetration of the leeward leg (punchthrough), or from sliding of the windward leg, due to low initial spud can (or mat) penetration; and/or (3) leg damage which usually occurs in the critical zone, at the leg/hull connection.
These three types of accidents are often closely linked. The essentially horizontal environmental loads (due to wave, wind and current) applied on the deck and legs of a unit in a jacked-up mode, generate a moment which tends to overturn the structure. Additionally, an excessive overturning moment may cause the sudden penetration of a leg which usually results in damage to the leg.
Leg members embedded in the soil with deep spud can penetration have an appreciable degree of lateral foundation resistance. For example, the increase in the overturning moment capacity may reach up to 40% in areas with at least 60 feet of penetration. This increase in the overturning moment is attributed to the lateral resistance of the chords and braces to movement in the surrounding soils. When the lateral foundation resistance is increased, the survivability of the jack-up rig improves. This increased survivability is especially beneficial in areas prone to revolving tropical storms such as the Gulf of Mexico and southeast Asia.
Therefore, there is a need to increase the lateral resistance of support members with an open lattice structure to movement in the surrounding soils. Specifically, there is a need to improve the overturning moment resistance of the legs of a jack-up drilling rig.